Add multi_field support for Mapper externalValue (plugins)

In context of mapper attachment and other mapper plugins, when dealing with multi fields, sub fields never get the `externalValue` although it was set.

Here is a full script which reproduce the issue when used with mapper attachment plugin:

```
DELETE /test

PUT /test
{
    "mappings": {
        "test": {
            "properties": {
                "f": {
                    "type": "attachment",
                    "fields": {
                        "f": {
                            "analyzer": "english",
                            "fields": {
                                "no_stemming": {
                                    "type": "string",
                                    "store": "yes",
                                    "analyzer": "standard"
                                }
                            }
                        }
                    }
                }
            }
        }
    }
}

PUT /test/test/1
{
    "f": "VGhlIHF1aWNrIGJyb3duIGZveGVz"
}

GET /test/_search
{
    "query": {
        "match": {
           "f": "quick"
        }
    }
}

GET /test/_search
{
    "query": {
        "match": {
           "f.no_stemming": "quick"
        }
    }
}

GET /test/test/1?fields=f.no_stemming
```

Related to elastic/elasticsearch-mapper-attachments#57

Closes #5402.

src/main/java/org/elasticsearch/index/mapper/DocumentMapper.java

@@ -248,10 +248,10 @@ public DocumentMapper build(DocumentMapperParser docMapperParser) {
     }
 
 
-    private CloseableThreadLocal<ParseContext> cache = new CloseableThreadLocal<ParseContext>() {
+    private CloseableThreadLocal<ParseContext.InternalParseContext> cache = new CloseableThreadLocal<ParseContext.InternalParseContext>() {
         @Override
-        protected ParseContext initialValue() {
-            return new ParseContext(index, indexSettings, docMapperParser, DocumentMapper.this, new ContentPath(0));
+        protected ParseContext.InternalParseContext initialValue() {
+            return new ParseContext.InternalParseContext(index, indexSettings, docMapperParser, DocumentMapper.this, new ContentPath(0));
         }
     };
 
@@ -484,7 +484,7 @@ public ParsedDocument parse(SourceToParse source) throws MapperParsingException
     }
 
     public ParsedDocument parse(SourceToParse source, @Nullable ParseListener listener) throws MapperParsingException {
-        ParseContext context = cache.get();
+        ParseContext.InternalParseContext context = cache.get();
 
         if (source.type() != null && !source.type().equals(this.type)) {
             throw new MapperParsingException("Type mismatch, provide type [" + source.type() + "] but mapper is of type [" + this.type + "]");